Many portable devices are battery powered. A user may carry several devices and sometimes the battery charge of one of the devices may drain. Some portable devices may be capable of receiving and transferring power through a bi-directional power transfer interface. One embodiment of such an interface is the USB Type-C. Bi-directional power transfer allows one device to donate battery charge to another device. Users having access to multiple portable devices thereby reduce the risk of a portable device running completely out of charge.